A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of one, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In current designs, a lack of wafer flatness and contamination on a wafer or generally on an object to be placed in a radiation beam, including but not restricted to substrates, wafers, fiducials and masks, negatively affects the imaging quality of the micropatterns that are imaged on target locations of the substrate. Likewise, contamination between the objects and supports that support the object while being irradiated can seriously impact imaging quality and cause deflection of the substrate.
Another problem affecting image resolution may be a damaged support structure, which, through wear or incidents, may have zones or specific spot locations that are not within specification for providing a flat support. However, the detection of such unflatnesses may be very difficult and time consuming.
Conventionally, a height map is used to drive the plane of focus of an imaging beam to provide well focused imaging at a desired height level. However, the height map created from a top surface of a (test) substrate can also be used to detect impurities as is illustrated in published European Patent Application Nos. EP1457828 or EP1093022. However, certain small impurities on the support structure can not be detected because they cause insufficient height variation. Yet, these impurities can cause distortion of the wafer which affects overlay accuracy and imaging quality.
U.S. Pat. No. 5,559,582 discusses a cleaning tool for cleaning impurities from a support structure. U.S. Pat. No. 6,198,982 discusses an impurity detection method by sensing a height deviation from a reference plane. Published Japanese Patent Application No. JP2000/252187 discusses placement of a wafer at a reduced clamping pressure level for mitigating the generation of dust.